1. Field of the Invention
The present invention relates to zoom lens systems, which are suitable for film cameras, video cameras or digital still cameras and the like.
2. Description of the Related Art
Recently, as the functionality of image-taking apparatuses (cameras), such as video cameras, digital still cameras or the like using a solid-state image-pickup element advances, zoom lenses having a large aperture ratio containing a broad field angle are desired for the optical systems used for such image-pickup apparatuses.
In this type of camera, various optical members such as a low-pass filter or a color correction filter are arranged between the rear end of the lens system and the image-pickup element, so that a lens system with a relatively long back focus is desired for the optical system used in such cameras. Furthermore, in the case of cameras using an image-pickup element for color images, a lens system with good telecentricity on the image side is desirable for the optical system used in such cameras, in order to avoid color shading.
Conventionally, various dual-unit zoom lenses also known as short-zoom wide-angle lenses have been proposed that are made of two lens units, namely a first lens unit having negative refractive power and a second lens unit having positive refractive power, and perform zooming by changing the distance between those two lenses. With such short zoom optical systems, zooming is performed by moving the second lens unit with positive refractive power, and the correction of the image position in the course of zooming is performed by moving the first lens unit with negative refractive power. In such lens structures made of two lens units, the zoom ratio is about 2×.
To achieve a lens system of an overall compact form having a zoom ratio greater than 2×, a so-called triple-unit zoom lens has been proposed in which a third lens unit having negative or positive refractive power is arranged on the image side of the dual-unit zoom lens, and various aberrations occurring with higher zoom ratios are corrected (see Japanese Patent Publication No. H07 (1995)-3507 (corresponds to U.S. Pat. No. 4,810,072) and Japanese Patent Publication No. H06 (1994)-40170 (corresponds to U.S. Pat. No. 4,647,160) for example).
Moreover, wide-angle triple-unit zoom lens systems are known that satisfy the requirements for back focus and telecentricity (see Japanese Patent Application Laid Open No. S63 (1988)-135913 (corresponds to U.S. Pat. No. 4,838,666) and Japanese Patent Application Laid Open No. H07 (1995)-261083 for example).
Furthermore, triple-unit zoom lenses are known, in which the first lens unit, which has negative refractive power, is immovable when zooming, and which perform zooming by moving the second lens unit, which has positive refractive power, and the third lens unit, which has positive refractive power (see Japanese Patent Application Laid Open No. H03 (1991)-288113 (corresponds to U.S. Pat. No. 5,270,863) for example).
Further known are triple-unit zoom lenses in which the number of constituent lenses is relatively small (see Japanese Patent Application Laid Open No. 2000-47108 (corresponds to U.S. Pat. No. 6,542,301), Japanese Patent Application Laid Open No. 2001-272602 (corresponds to U.S. Pat. No. 6,545,819), Japanese Patent Application Laid Open No. 2002-14284, Japanese Patent Application Laid Open No. 2002-48975 (corresponds to U.S. Pat. No. 6,417,973), Japanese Patent No. 3035830 (corresponds to U.S. Pat. No. 5,270,863), and U.S. Pat. No. 5,278,698 for example).
Among these conventional examples, Japanese Patent Application Laid Open No. 2000-47108, Japanese Patent No. 3035830 and U.S. Pat. No. 5,278,698 disclose that the first lens unit is configured by two lenses, namely a negative lens and a positive lens, that the refractive index of the material of the negative lens and the positive lens is kept relatively low, thereby achieving both the correction of chromatic aberration and image plane correction.
In triple-unit zoom lenses that are designed for 35 mm films, the back focus is long and their telecentricity is not very good, so that it is difficult to use them directly in optical apparatuses having solid-state image-pickup elements.
In recent years, in order to make cameras more compact while at the same time increasing the zoom ratio of the zoom lens, so-called collapsible zoom lenses are widely used, in which the distance between the lens units when not image-taking is contracted to a distance that is different of the distance when image-taking, and in which the lens protrudes from the camera main body only by a small amount.
Ordinarily, when the number of lenses of each lens unit constituting the zoom lens is large, the length of the lens unit on the optical axis becomes long, and when the amount that each lens unit is moved during zooming and focusing is large, then the overall length of the lens system becomes large and the desired collapsed length cannot be achieved, making it difficult to use a lens system for a collapsible zoom lens.